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04-13-2012, 05:56 PM   #31
MLXXXp
Enthusiast

Posts: 37
Karma: 88
Join Date: Dec 2009
Location: Toronto, Canada
Device: Aluratek Libre, Nokia N800
Quote:
 Originally Posted by MLXXXp With the right equipment, you don't have to get the Libre into a known charge state or do any timing. Just use a voltmeter and an ammeter (preferably at the same time, or alternatively a wattmeter) to measure the voltage and current (or power) going to the Libre, from the output of the adapter.
In case Ken, or anyone else, is still interested in this topic, I've done some work to determine the power requirements for charging an Aluratek Libre.

I built a Mini-USB breakout box to allow me to measure the current that the Libre is drawing while charging.
Details of the box are at this link

Here's a link to a picture of my measurement setup

The device with the large black heat sink, at the lower left, is a DC voltage regulator circuit that I built. It's being used to provide a nice stable voltage (I set it for 13V) to a lighter socket. The regulator is being fed from a 15VDC unregulated "wall wart" adapter that's not in the picture.

A white 12V to 5V USB adapter is plugged into the silver lighter socket to the right of the regulator. The adapter is rated maximum 1A output, 12-24V input. The meter at the far left is measuring the voltage on the lighter socket (thus the adapter input) and the meter next to it is measuring the current drawn by the adapter.

The USB cable that came with the Libre is connected between the adapter and my break out box, and the box's plug goes to the Libre. The beige Beckman meter is measuring the voltage on box's plug cable (thus the Libre's input) and the meter to the right of it is measuring current drawn by the Libre.

During these measurements, the Libre's battery was quite low.

Calculating power to the Libre:
4.81V X 0.54A = 2.60 Watts.

Calculating power to the Adapter:
13.00V X 0.28A = 3.64 Watts.

Calculating the efficiency of the adapter:
2.60W / 3.64W = 0.71
(The adapter has a conversion efficiency of 71%)

For use with a 12V solar panel, the power into the adapter (overall power) is the important value. To operate in this set up, in place of the regulator, a panel would need to be able to provide 3.64 Watts. (A panel capable of providing more power wouldn't hurt, but it wouldn't speed up charging any.)

Now, if you look at the AC adapter that came with the Libre, you'll see a diagram that indicates that the negative output goes to both pins 4 and 5 (pin 4 is what I call pin X for my break out box). In other words, pin X is grounded.

I moved the pin X jumper on my box so that pin X was grounded.
Here's a link to a picture of this setup

Notice that the Libre is now drawing more current, since it sensed that pin X is grounded and thinks that a high power charger is attached.

Calculating power to the Libre:
4.51V X 0.84A = 3.79 Watts.

Calculating power to the Adapter:
13.04V X 0.43A = 5.61 Watts.

Calculating the efficiency of the adapter:
3.79W / 5.61W = 0.68
(Here, the adapter has a conversion efficiency of 68%)

So if you had a way to ground pin X (like I do) and wanted to use a solar panel, the panel would have to be able to provide 5.61 Watts.

If your 12V to USB adapter was more efficient than mine, your panel power requirements would be reduced proportionately. And actually, there's a very small amount of power lost in the ammeter measuring the Libre current, so slightly less power would be required if the meter is removed.

Last edited by MLXXXp; 04-13-2012 at 06:04 PM.